Service engine light comes on up grades

[cranky old fart]

Hi Guys,
My service engine soon light comes on when I start to pull a grade. It happens around 50 mph at about 2000 rpm .When the grade levels off it goes out .I'm not pulling hard or towing .I've only had the truck ,'94 2500 6.5 W/176,000 miles,a couple months, but it didn't do it when I bought it. Any ideas?
Thanks,Jeff

[JohnC]

First check the DTC codes. My bet is you'll find DTC 78. then sheach these forums for DTC 78 (or whichever one you come up with...). It's been covered in great depth.

[Jim Faire]

First check the DTC codes. My bet is you'll find DTC 78. then sheach these forums for DTC 78 (or whichever one you come up with...). It's been covered in great depth.

Yep... you appear to have a boost problem. Easy to fix.. check for codes, like John said. You know how to check for your codes without a scanner?

[cranky old fart]

No ,I don't know how to check codes w/o a scanner. Please, do tell.

[93GMCSierra]

paperclip into the last 2 holes on the top of the computer port, usually under the dash near the steering wheel, then turn the key to on position then watch the service engine light, it will flash, once, pause then twice, code 12 test code then it will repeat a few times, then start a new code then repeat the entire cycle till you turn off the key, atleast that is how my 86 chevy works

[Patrick m.]

then check for vac at idle at the wastegate actuator. if no vac, trace plastic tube probably cracked somewhere.

[cranky old fart]

OK,
So I jumped the computer port. The service light flashed 7 times then 8 .Dtc 78?
Thanks, Jeff

[cranky old fart]

OK again ,
I did a search for dtc 78 but didn't get a clear direction for the fix. Can someone help? Jim?
Thanks

[Jim Faire]

OK Jeff, you got yerself a wastegate issue. That means your turbo isn't working right... could be lots of things.

I would suggest you order yourself a TurboMaster from Heath Diesel ... it's easy to install with basic hand tools, comes with instructions, and it will make things right again.

After that, we can talk about removing your vacuum pump.

Oh, you want to test it some more? If you can move the turbo wastegate arm at idle with your fingers, you have vacuum issues.

Get a vacuum gauge and start testing lines, leading to the wastegate solenoid located above the driver's side valve cover. You should have 18-22 psi on the line leading TO the solenoid (from the vac pump) at idle, and then you should have at least 12 psi at the wastegate actuator line (at the turbo) at idle.

If you have lots of vacuum before the solenoid, and significantly less at the turbo, either the line has a hole in it or you need a new solenoid.

If there is not much vacuum before the solenoid, you need a new vacuum pump.

NOBODY puts new vac pumps on an F engine. Get a Turbomaster.

As a matter of fact, get a TurboMaster anyway. You'll like it.

[DennisG01]

Well, now c'mon. Don't say "nobody". ;)

If my vac pump ever goes, I'll replace it with a new one. The particular ECM program I have from Kennedy is designed to specifically work with the vac system. There's absolutely nothing wrong with a TM - it's a great idea. But after to listening to John Kennedy talk to me about the advantages of the stock system, I chose to stick with it. It all depends on what you want - I will admit, though, for most people the TM is the way to go.

[Warren96]

I replaced mine.The computer controled stock system has some good safegaurds built in. Might even be a lot cheaper.

[Robyn]

The Turbo master is a great little device but without a Chip in the ecm you can end up with continued codes.

Let me advise on how to diagnose the issue.
Check the vacuum at the vacuum pump (Front RH side of the engine)
Vacuum should be 26" at the pump. If the vacuum gauge wobbles all over or is low toss the pump and replace it.
Now if the vacuum is good trace the plastic line to the wastegate solenoid on the LH Valve cover. Be sure the vacuum to the solenoid is the same as the pump.
Now if this is good check the vacuum at the wastegate actuator on the turbo. The vacuum here may be slightly lower. If there is zero or very little vacuum there be sure the plastic lines are good.
If the lines all check and the vacuum is bad to the turbo then its most likely the solenoid on the VC thats bad.
The solenoid is noted for failing.

Now let me ask you this, when the light comes on and your pulling the hills, does the rig smoke heavily out the tail pipe?? If it does then you have a low/ no boost situation. If there is no smoke you may have a stuck wastegate that is allowing too much boost. Either too much or not enough will set a 78.

Check it and let us know.

Robyn

[JohnC]

My experience suggests that the earlier trucks will NOT set a code on low boost. I base this on the fact that I drove arounnd for 2 weeks with a dead vacuum pump and never set a code.

The common failure mode for the ODB1 trucks is for the solenoid to get sluggish. It then cannot bleed off the vacuum precisely enough to prevent OVER boost under load. The PCM detects the over boost condition and stops pulsing the solenoid, which eventually bleeds off all the vacuum, resulting in under boost, which is the symptom, not the cause.

YMMV

[Robyn]

John C
This is interesting. The other 94 I had would set a code 78 all the time when I first got it off the used car lot. It had about 6" of the plastic vacuum line gone between the Pump and the solenoid.
Stopped at a little parts store on the way home and fixed it with a piece of rubber hose and no more issues.
It would pop the 78 as soon as I would throttle up on a hill and it would blow enough smoke to take care of a swamp full of skeeters. :D

I think it can go either way. Now my dually with the turbo master would pop the 78 as soon as I got on it for more than 15 seconds with boost over about 6 PSI.

The new chip fixed all that and I can hold it hard on 10-12 until the cows come home and no more issues.

best

Robyn

[cranky old fart]

So I found the vac. pump under the a/c compressor. A rubber vac. hose comes from below then a white plastic tube (small) attaches to it and goes to the set of 3 solenoids on the driver side valve cover then to the waste gate actuator.I have very little vacuum at the solenoid. Does the plastic tube just push into the vacuum hose coming from the pump? Didn't want to break it! Also is it ok to drive it in this condition ,just light duty? Will it hurt any thing to leave the coweling off for awhile
Jeff

[DennisG01]

Other than not having full power, you won't hurt anything.

Cowling? Do you mean the gray "6.5 Turbo" cover? Take it off and leave it off. It serves absolutely no purpose (other than aesthetic). In fact it might actually be somewhat detrimental as it could trap heat.

[JohnC]

Maybe '94s are different... 3 psi was the most I could get wth the bad vacuum pump. The light never came on... I had to tow once before the new pump came. Heres my workaround...

http://www.skyportservices.net/images/stuff/turbo-bastard.jpg

(Note that is genuine used baling wire.)

[93GMCSierra]

Wow, that looks suspiciously like my extra boost controller on my 93...lol

[cranky old fart]

Hey Jim, Robyn, and all,
Thanks soooo much. The vacuum line splitter or "Y" was cracked at the solenoid bank. A few inches of vacuum hose and 45 cents later and all is well.
Thanks again,
Jeff